Two-Step Low-Pressure Direct Injection System for Hydrogen Fuelled Engines

The paper describes the CFD analysis, the arrangement and the first experimental results of a single-cylinder engine that employs an innovative low-pressure hydrogen directinjection system, characterised by low fuel rail pressure (12 bar) and consequent low residual storage pressure. The injection is split in two steps: at first hydrogen is metered and admitted into a small intermediate chamber by an electroinjector (a conventional one usually employed for CNG), next a mechanically-actuated poppet valve, that allows high volumetric flow rates, times hydrogen injection from the intermediate chamber to the cylinder within a short time, despite the high hydrogen volume due to the low injection pressure. Injection must be properly timed to maintain pressure below 6 bar (or little more) in the intermediate chamber and thus keep sonic flow through the electroinjector, to maximize volumetric efficiency and to avoid backfire in the intake pipe. A preliminary CFD study based on code KIVA3v was carried out in order to compare the behaviour of different injection valve and valve-seat geometries in controlling air-fuel mixing inside the cylinder and to address the choice towards the most suitable ones, i.e. those that allow adequate fuel distribution in the combustion chamber at ignition time for every operating condition. As well, all the modifications necessary to obtain the prototype engine from a production one were based on KIVA3v predictions, especially as regards the volume of the intermediate chamber and diameter, maximum lift and opening duration of the injection valve. The prototype derives from the single-cylinder Aprilia-Rotax 650 cm3 with five valves (three intake and two exhaust). The cylinder head underwent deep changes to replace the central inlet valve and pipe with the ones for hydrogen injection. Special attention was paid to crankcase ventilation system to prevent the formation of hydrogen pockets. Experimental results show the effectiveness of the solution. The engine proved to run correctly, without the typical drawbacks of hydrogen engines (preignition, knocking, backfire, roughness) even with stoichiometric or slightly rich mixture (for maximum power). Maximum power was higher than for the engine fed with gasoline. A new CFD study based on AVL_Fire code recently started taking engine actual details into account with the aims to better predict engine behaviour and to address the improvement of the whole system.